Treatment of intergranular sulfur corrosion in metals



United States Patent 3,287,181 TREATMENT OF INTERGRANULAR SULFURCORROSION IN METALS Bernard Steverding, Huntsville, 'Ala., assignor tothe United States of America as represented by the Secretary of the Army,7 No Drawing. Filed Nov. 7, 1963, Ser. No. 322,256 12 Claims. (Cl.148-203) The invention described herein may be manufactured and used byor for the Government for governmental purposes without the payment ofany royalty thereon.

The present invention relates to a treatment for'prevention ofintergranular corrosion in metals as well as rejuvenation of metal whichhas been effected by intergranular sulfur corrosion.

In the past much diificulty has been encountered in maintaining thedesired characteristics of metal when the metal has been exposed tosulfur containing vapors at elevated temperatures.

This phenomenon is especially troublesome in missile thrust chamberswhere the sulfur content of the fuels used is about 0.03 to about 0.05percent by weight. To further reduce the sulfur content of these-fuelswould be impractical because of the excessive cost of .this type ofpurification. To provide a nozzle cooling means in liquid propelledmissile systems, the missile fuel is cycled through a multitude of smalltubes contained in the walls of the thrust chamber. Nickel andnickel-base alloys are commonly used for the construction of thrustchambers and under normal conditions these alloys are quite resistant togranular corrosion of any type.

Under certain uncontrollable conditions small superheated areas mayoccur within the thrust chamber cooling tubes in the vicinity of'thenozzle throat. The temperature at these so-called hot spots is oftenhigh enough to cause local evaporation of the fuel and the sulfurcontained in the fuel vapor then becomes extremely active as aninitiator of intergranular corrosion.

Other metals, of course, may be used in such a system and would beeffected in a similar manner to a greater or lesser extent. 7

The thrust chamber is often used repeatedly in ground tests andtherefore must be very durable to achieve satisfactory testing ofmissiles. Because corrosive attack within the tubes is very difiicult todetect, it will be seen that the probability of failure because ofintergranular corrosion caused by sulfur becomes even greater when thenozzle must be reused. The problem, therefore, becomes not only one ofprevention of intergranular corrosion but also one of rejuvenation ofpreviously fired thrust chambers. This rejuvenation should be possiblefor the most severe sulfur attack, as long as mechanical fracture doesnot occur.

Accordingly, an object of my invention is to provide a treatment whichwill restore the original characteristics of metals which have beeneffected by intergranular sulfur corrosion.

Another object of my invention is to provide a treatment ofintergranular sulfur corroded metals which will be simple and economicalwhile consuming a relatively small amount of time.

Still another object of my invention is to provide a treatment whichwill prevent sulfur corrosion in metals.

Much experimentation has been performed in an effort to alleviate thedeterioration of metals which have been exposed to sulfur undercorrosive conditions. Probably the worst defect of metals caused bysulfur corrosion is the sulfur build-up between the grains. Thisphenomenon is referred to as intergranular corrosion and oftenpenetrates the metals to extensive depths. This infiltration 3,287,181Patented Nov. 22, 1966 by sulfur between the grains of metals weakensthe binding connection between the grains thus causing extremebrittleness, as well as loss of the metals most desirablecharacteristics of strength and toughness. In order to remove the sulfurfrom the grain boundaries the inventive treatment contemplates heatingthe metal in a vacuum to the point at which the sulfur coagulates intospheroidal particles which tend to relocate away from the grainboundaries. The original grain binding characteristics of the metal arethus restored. The temperature of the treatment should be kept below therecrystallization tern perature of the metal so that no grain growthneed occur.

It has been found that once treated in this manner a metal becomes agreat deal less susceptive to further intergranular sulfur corrosion.Thus, for maximum protection a metal would be subjected to conditionscausing intergranular corrosion, and then treated so as to eliminate thecorrosion. By this method the metals resistance to further corrosionwould be greatly increased.

Experimentation has shown that similar results occur and the decrease inpressure need not be so great when the metal is heated in an atmosphereof hydrogen. However, to avoid oxidation, hydrogen of great purity mustbe used if the treated metal is an alloy containing chromium.

It has been found in the case of nickel which is commonly used in thrustchambers, that satisfactory results are obtained by vacuum firing themetal at temperatures between about 500 C. and about 1100 C. at anabsolute pressure of between about 10- mm. Hg and about 10* mm. Hg for aperiod of between about 1 hour and about 48 hours, while excellentresults have been achieved 10- mm. Hg and 10* mm. Hg for a period ofbetween 10 and 18 hours.

During this study one nickel sample was severely corroded by exposure toH S vapor at 600 C. for onehalf hour. The rejuvenation treatment wasperformed at a pressure of 10 mm. Hg and a temperature of 700 C. for 14hours. The results of this treatment were excellent. The corroded samplehad been completely embrittled because of infiltration by the sulfurinto the grain boundaries. After the restoration treatment the sulfurhad coagulated into small, well distributed particles well away from thegrain boundaries. Further tests revealed that the sample had regainedits original characteristics of strength, ductility and toughness, whileits resistance to further intergranular corrosion hadbeen greatlyincreased.

Further study has indicated that for other metals such as zirconiumalloys, steel, super alloys and transition metals the temperature rangemust be varied as well as the pressure and time. The time, for example,may range up to about 48 hours, the temperature variation may be fromabout 200 C. to about 1500 C. and the pressure may range from about l0mm. Hg to about 10" mm. Hg. For most metals of the type used in thrustchambers results were good when using a temperature of between 600 C.and 1200 C., a pressure of between 10- mm, Hg and l0 mm. Hg for a periodof between.

3 The treatment would thus be useful in laboratories not equipped toreduce pressures to the limits necessary when utilizing a normalatmosphere.

Thus, it will be seen that a metal which has been previously afiected byintegranular sulfur corrosion canbe rejuvenated by vacuum firing.Seconadly and even more important is the fact thatthe meal is made moreresistive to such corrosion than it was in its original state. As afinal step toward even further resistance to intergranular corrosion themetal may be cold rolled. This treatment will cause the grain boundariesto run more closely parallel to the surface of the metal therebydecreasing the depth of the lateral grain boundary which in turn reducesthe depth to which the corrosion may advance.

It is thus within the concept of my invention that in addition to thetreatment of thrust chambers presently in use the treatment can be inthe fabrication of new thrust chambers. In the treatment of the newmaterials of fabrication the materials would be subjected to anintentional exposure to sulfur gases before being vacuum fired.

While the foregoing is a description of the preferred embodiment, thefollowing claims are intended to include those modifications andvariations that are within the spirit and scope of my invention.

I claim:

1. A'method of treating intergranular sulfur corrosion in nickel andnickel base alloys which comprises heating the metal in an atmosphere ofhydrogen to a temperature of between about 500 C. and about 1100 C. at.a pressure between about .5 and about .1 atmosphere for a period of timeof between about 1 hour and about 48 hours.

2. A method of treating intergranular sulfur corrosion in metals whichcomprises taking said metal after it has been exposed to a sulfur vaporenvironment suflicient to cause intergranular sulfurf corrosion in saidmetal andheating said metal to a temperature of between about 200 C. andabout 1500 C. at a pressure between about 10- mm. Hg and about 10" mm.Hg for, a period of not more than about 48 hours.

3. A method of treating intergranular sulfur corrosion in metals whichcomprises taking saidmetals after they have been exposed to a sulfurenvironment sufiicient to cause intergranular sulfur corrosion in saidmetals and heating said metals in an atmosphere of hydrogen to atemperature of between about 200 C. and about 1500 C. at a pressure thatis maintained between about .5 and about .1 atmosphere for a period ofnot more than about 48 hours.

4. The method of treating intergranular sulfur corrosion in metals whichcomprises taking said metalsafter they have been heated to an elevatedtemperature and exposed to sulfur vapor and heating said metals to atemperature between about 600 C. and about 1200 C. at a pressure ofbetween about 10* mm. Hg and about 10- mm. Hg.

and exposed to sulfur vapor and heating said metals in 5 an atmosphereof hydrogen to a temperature between about 600 C. and. about 1200 C. andat a pressure that is maintained between about .5 and about .1atmosphere.

6. A method of making-metals resistant to sulfur corrosion comprising;exposing said metals to sulfur vapor, heating said metals to atemperature of between about 650 C. and about 800 C. ate pressure .ofbetween about mm. Hg and about 10- mm. Hg for a period of time ofbetween about 10 hours and about 18 hours.

7. A method of making metals resistant to sulfur corrosion comprising;exposing said metals to sulfur vapor, heating said'me-tals in anatmosphere of hydrogen to .a temperature of between about 650 C. andabout 800 C. at a pressure of between about .5 and .1 atmosphere for aperiod of time of between about 10 hours and about:

18 hours. r

8. A method as set forth in claim 7 wherein said metal is cold rolled asa final step.

9. A method of making metals resistant to sulfur corrosion comprising;exposing said metals to sulfur vapor, heating said metals to atemperature of between about 500 C. and about1l00 C. at a pressure offrom about 10- mm. Hg to about 10 mm. Hg for a period of time of betweenabout 1 hour and about 48 hours.

10. A method as set forth in claim 9 wherein metals are nickel andnickel base alloys.

11. A method of making metals resistant to sulfur corrosion comprising;exposing metals to sulfur vapor, heat-.

ing said metals to a temperature of between about 200 maintained betweenabout .5 and about .1 atmosphere for a period of not more than about 48hours.

12. A method as set forth in claim 11 whereinsaid.

metals are nickel and nickel base alloys.

References Cited by the Examiner UNITED STATES PATENTS 1,201,633 10/1916Ruder 148-16 1,842,200 1/1932 R-amage 148-20.3

2,314,816 3/1943 Brown et a1 148-20.3

2,662,839 12/ 1953 Becker et a1 148-20.3

OTHER REFERENCES Metals Handbook, 1948 edition, The American Society forMetals, Cleveland, Ohio, relied on page 1042.

HYLAND BIZOT, Primary Examiner. 7 DAVID .L. RECK, C. N. LOVELL,Assistant Examiners.

said

C. and about 1500" C. in an atmosphere of hydrogen

9. A METHOD OF MAKING METALS RESISTANT TO SULFUR CORROSION COMPRISING;EXPOSING SAID METALS TO SULFUR VAPOR, HEATING SAID METALS TO ATEMPERATURE OF BETWEEN ABOUT 500*C. AND ABOUT 1100*C. AT A PRESSURE OFFROM ABOUT 10-**3 MM. HG TO ABOUT 10-**6 MM. HG FOR A PERIOD OF TIME OFBETWEEN ABOUT 1 HOUR AND ABOUT 48 HOURS.